This invention relates generally to transmitting antennas of electromagnetic radiation and more particularly to active transmitting antennas.
In State of the art VHF antennas in the form of a center-fed dipole, being, for example, six feet in height and having a ten band tuning and matching network in its base, such an antenna has been found to be very efficient; however, if one tries to reduce the size of the antenna to less than 0.1 of its operating wavelength .lambda., several deleterious effects arise, namely the bandwidth of the antenna decreases, the real or resistive part of the input impedance decreases while the reactive or imaginary part increases drastically, and radiation inefficiency decreases because the tuner losses increase due to the increase of the reactive component of the impedance. Also the complexity of the tuning network increases appreciably. Since the antenna is normally coupled to a transmitter of 50 ohm nominal impedance, a matching network capable of a 10:1 to 20:1 impedance transformation is required over a 2:1 to 3:1 bandwidth. Moreover, if high powers, such as 75 watts or more are used, the tuning and matching networks in these high Q configurations undergo severe voltage stress. If losses are introduced to ease the bandwidth problem such a step results in severely reduced radiation efficiency. In brief, the provision of a low profile broadband antenna of a relatively simple design capable of radiating 75 to 100 watts efficiently has proved to be extremely difficult if not impossible to obtain.
While active antennas are generally known, the use of short active dipoles for receiving purposes has not been followed by corresponding advances for transmitters primarily because reciprocity no longer applies and therefore different configurations are required for both transmitting and receiving applications. One type of active transmitting antenna has been disclosed in an article entitled "Short Range Active Transmitting Antenna With Very Large Height Reduction", by T. S. M. MacLean, et al. which appeared on March, 1975 in the IEEE Transactions On Antennas And Propagation, at pages 286 and 287. The configuration disclosed therein comprises a transmitting system having an antenna whose dipole height is of the order of .lambda./2000 and is of the form called the fed-emitter base loop configuration which includes a transistor coupled between the antenna and the source and ground plane. Another type of active transmitting antenna has been disclosed in an article entitled, "Low Profile UHF Antenna", appearing in Electronics World at page 65 on June, 1966. In that configuration, a .lambda./4 length of wire is formed into a figure "8" configuration which is mounted 2.5 electrical degrees above a ground plane and which is fed from a transistor located beneath the ground plane.
While the known prior art purportedly operates as specified, it has nevertheless been found extremely difficult to both tune and match the antenna of these relatively small heights or sizes over a 2:1 or 3:1 frequency band without RF loss and without electrical and/or mechanical element variation.
Accordingly, it is an object of the present invention to provide an improved high power RF active transmitting antenna which is operable over a relatively wide frequency range.
It is another object of the present invention to provide a high power RF antenna which exhibits a low profile.
And yet another object of the invention is to provide an active transmitting antenna wherein the circuit interface between the antenna and transmitter is substantially minimized or eliminated.